1. Field of the Invention
This invention relates to a method and system for fueling hydrogen-fueled vehicles including internal combustion engine and fuel cell powered vehicles. More particularly, this invention relates to a method and system for controlling high-pressure hydrogen dispensers in fast-fill fueling stations to ensure complete, safe filling of vehicle on-board hydrogen storage vessels, regardless of station hydrogen supply gas and ambient conditions.
2. Description of Related Art
The on-board storage capacity of hydrogen powered vehicles, both internal combustion engine and fuel-cell powered, defines the driving range allowable before refueling is required. Underfilling of hydrogen vehicle storage vessels, during fast fill (considered to be less than about 5 minutes) charging operations, can occur at fueling stations having dispensers which incorrectly or inaccurately compensate for initial vehicle hydrogen storage vessel pressures, station supply gas conditions, and ambient temperatures. At higher ambient temperatures, i.e. greater than about 95° F., this underfilling can reach 20% or more of the rated gas mass storage capacity of the storage vessel. This underfilling represents a serious marketing obstacle to be overcome by the hydrogen supply industry, without resorting to unnecessarily high fueling station gas and vehicle storage pressures, which can increase station manufacturing costs and vehicle weights.
Undercharged vehicle hydrogen storage vessels during fast fills are partially the result of the fueling station dispenser either ignoring, or inaccurately estimating, the elevated hydrogen storage vessel gas temperatures that occur in the charging period due to compression, mixing, heat transfer to vessel walls, and other complex and transient thermodynamic processes.
During charging, the expansion of the hydrogen gas that occurs when flowing from the station ground storage reservoirs, or directly from the station compressor, does not reduce hydrogen gas temperature, as occurs, for example, in natural gas dispenser control systems, because hydrogen has a reverse Joule-Thomson effect. That is, in throttling gas flow processes, hydrogen gas temperature actually increases at the lower downstream pressures. This causes a further increase, relative to natural gas, in the transient gas temperature within the vehicle hydrogen storage vessel while it is rapidly filled. This, in turn, decreases the density of the hydrogen gas, causing severe underfilling, if not compensated for in the fill algorithm, for the same transient storage gas pressure.
The end of the charge process is often terminated when the fueling station dispenser computer measures, or estimates, that point at which the vehicle hydrogen storage vessel reaches a certain level of pressure. Depending on the dispenser, this level of vessel cut-off pressure may have some dependency on ambient or station gas conditions, but no known existing hydrogen dispensers compensate for the vessel gas temperature rise during charging as well as for initial vessel and station gas supply conditions.
The temperature that the vehicle hydrogen vessel gas reaches at the end of the dynamic charging process is difficult to accurately estimate or measure and utilize in a dispenser fill and control methodology. As previously mentioned, hydrogen storage vessel temperature is a complex function of several fueling station and initial vessel gas conditions, as well as vessel ambient and dynamic gas-to-vessel wall heat transfer conditions. Testing has shown this temperature to be non-uniform at the end of the fill.